Ontogenetic variation in the diet of the cleaner fish Labroides dimidiatus and its ecological consequences

Ontogenetic changes in the diet of the cleaner wrasse Labroides dimidiatus (Labridae) were examined at Lizard Island, Great Barrier Reef. Cleaner fish of all sizes mainly ate gnathiid isopod juveniles; the proportion of these (77 to 85%) did not differ among size classes. However, the proportion of the remaining items (scales, parasitic copepods, and non-parasitic copepods) differed significantly among size classes largely due to small juveniles which ate more non-parasitic copepods. The number of gnathiid isopods and scales in the diet of L. dimidiatus increased with the size of fish, with adult cleaners having 7 times as many gnathiids and 4 times more scales compared to small juveniles. The size-frequency distribution of gnathiids differed among size classes of cleaners; small juvenile cleaners had more small gnathiids in their diet than medium-sized juveniles and adult cleaners. The 'throat width' increased with the size of cleaner; on small juvenile cleaners the throat width was equal to or less than the width of the larger gnathiids sampled on a common client fish Hemigymnus melapterus (Labridae). Mouth size constraints may, therefore, explain the low number of large gnathiids found in the diet of these small cleaners. The lower number and smaller size of gnathiids in the diet of juvenile cleaners compared to adult cleaners suggests that any potential impact of small cleaner fish on gnathiids abundance will be less than that of larger cleaners and will be largely on small gnathiids. In addition, although clients may benefit more from cleaning by the larger cleaners, because they remove more parasites, the costs in the form of loss of scales, which are also removed in larger numbers, will also be higher